According to certain standard specifications, EC motors of the above-referenced type must be protected against overheating in the event of abnormal operating conditions.
To that end, it is known to place so-called temperature sensors inside the motor in the area of the stator coils, which as temperature-dependent switches, are triggered upon reaching a certain temperature limit and the operating current is usually directly or indirectly shut off by means of a trigger element. With certain motors, e.g., with external rotor motors, it is expensive both with regard to construction and manufacturing technology to house temperature sensors in or on the stator coil so that they can sense rapid temperature increases, as they can occur in particular with blockages (stalling) of the motor, and be able to safely shut off the motor.
It is further known how to design motors with impedance protection, i.e. to dimension the coil resistance and the wire diameter such that no overheating can occur during abnormal operation. A design with impedance protection is not possible in particular for EC motors with single tooth windings, however, because the winding resistances are very small.
Therefore up until now, a software controlled current limitation was integrated into the commutation control in which the software control specifies a lower threshold for the starting current and an upper threshold for the operating current. If the lower threshold is met or exceeded during the start-up phase, then the motor is shut off. However, if the motor runs normally, then a monitor of the operating current is commutated with regard to the upper threshold, and if the upper threshold is met or exceeded, the motor is shut off.
Such protective measures that can be implemented solely by software are not considered however by certain accreditation agencies, such as for example UL (Underwriters Laboratories), during the tests for the award of certain marks of approval.
The object of the present invention is to create a control circuit of the aforementioned type which guarantees an increased operational reliability for the prevention of motor overheating.
Accordingly the hardware design of the inventive control circuit comprises two redundant stall protection units which monitor the motor during operation, i.e., with the presence of driver signals for the driver stage upon the turning of the rotor, whereby in the case of a determined stoppage, the first stall protection unit deactivates the driver stage and the second stall protection unit shuts of the supply voltage for the driver stage.
By means of the inventive design, the sought-after high operational certainty is attained because, if the software current limitation control should fail for certain reasons, a secure shutting-off of the motor in an abnormal operating condition is guaranteed by at least one of the two redundant stall protection units.
Furthermore in another advantageous feature of the invention, two redundant, hardware designed, excess current protection units are provided which monitor the motor operating current, whereby in the event that the first current limiting value is exceeded, the first excess current protection unit deactivates the driver stage and in the event that the second current limiting value is exceeded, the second excess current protection unit shuts off the supply voltage for the driver stage. It should be preferably provided in an adaptation to the implied software current limitation in the commutation control that the first current limiting value of the first excess current protection unit is slightly greater than an upper threshold value of the software current limitation for the operating current, while the second current limiting value of the second excess current protection unit is slightly greater than a lower threshold of the software current limitation for the start-up current. In this way, if the electronic software current limitation should fail, shut-off occurs during start-up at the latest upon reaching or exceeding the second current limiting value, and during operation, at the latest upon the reaching or exceeding the first current limiting value. To that end, the second excess current protection unit in operation is deactivated during a determined cycle of the motor, and indeed preferably via an operating signal of one of the two inventive stall protection units.
In the following the invention will be explained more in detail using a preferred embodiment.